Magnetic heating in extrusion apparatus



Sept. 15, 1959 F. N. RoTHAcKl-:R

MAGNETIC HEATING 1N ExTRUsIoN APPARATUS Filed Sept. 25, 1957 Rei- INVENTOR FRA/vals Ns/LL Pan/A cKfA ATTORNEY nited States Patent O MAGNETIC HEATING 1N EXTRUSION APPARATUS Francis Neill Rothacker, East Orange, NJ., assignor to Sealtron Corporation, Long Island City, N Y., a corporation of Delaware Application September 25, 1957, Serial No. 686,116

3 Claims. (Cl. 219-10.49)

The present invention relates generally to improvements in apparatus for the extrusion of thermoplastic material, and it relates more particularly to an improved system for heating andcontrolling the temperature of the thermoplastic material.v

In accordance with the conventional methods of forming extruded shapes of synthetic organic thermoplastic material, various types of extrusion devices are employed. These generally include an elongated tubular barrel with which registers a rotating worm of successively smaller pitch which advances the plastic from the feed end of the barrel to the extrusion end thereof. A hopper is located at the feed end of the barrel and communicates with the interior thereof and a heated extrusion die having the desired opening is located at and communicates with the discharge end of the barrel.

In order to effect the satisfactory extrusion of the thermoplastic material, the temperature thereof along the full length of the barrel and at the extrusion die must be very closely controlled. The proper control of the temperature of the thermoplastic material has presented a major problem and the absence thereof accounts for an inferior and rejected product and non-productive out-time of the extrusion equipment. The usual methods for heating the extrusion barrel are either with circulating steam or hot oil or electrically. The steam and hot oil methods are expensive and diiiicult and of very little ilexibility. The conventionalv method for electrically heating the extruder barrel employs electrical resistance heaters of various types which are located along the length of the barrel. The currents to these heaters are generally regulated to maintain the desired temperatures at the various extruder barrel zones. In many cases, however, the temperature of the. extruded material is raised by reason of the energy delivered thereto as a result of the screw frictional losses ordue to exothermic reactions so that cooling is required to maintain the optimum temperatures. The cooling is usually accomplished by circulating a heat exchange medium in heat vexchange contact with the extruder barrel between the heaters and the interior of the barrel.

' `It is apparent from vthe above that the heating and temperature control o'fan extruder barrel is at its best a cost- 1y, Complicated, high power consuming operation requiring expensive: equipment and highly trained personnel. This is particularlytrue since the diiferent plastics and operating conditions require different temperature distributions. Furthermore, the conventional electrically heated extruder barrels are generally characterized by the development of hot spots yor uneven heating and by very slow cooling.

' Another drawback of the conventional plastic extruding machines isy the problem of effecting a free flow of the pellets, through the k'hopper into the barrel. By reason of the heatingof the hopper by conduction from the hot barrel, the plastic granules or pellets tend to adhere to the hopper'walls and` impede the ow thereof into the barrel,

VPatented Sept. 15, 1959` It is, thus a principal object of the present invention to provide an improved extruder apparatus.

Another object of the present invention is to provide an improved apparatus for the extrusion of synthetic organic thermoplastic materials.

Still another object of the present invention is to provide a plastic extruding apparatus having an improved plastic heating system.

A further object of the present invention is to provide a plastic extruding apparatus having an improved extruder barrel temperature regulating system.

Still a further object of the present invention is to provide an improved plastic extruder having a heating and temperature control system characterized by its ruggedness, simplicity, versatility, high eciency and low power consumption.

Another object of the present invention is to provide an improved plastic extruder characterized by controlled uniform heating of the plastic and the simple and rapid cooling of the extruder barrel.

Still another object of the present invention is to provide an improved plastic extruder so constructed and provided with a heating system that the free flow of plastic material into said extruder is unimpeded.

The above and further objects of the present invention will become apparent from a reading of the following description, taken in conjunction with the accompanying drawings, wherein Figure l is a schematic and longitudinal sectional view of the improved extruding apparatus; and

Figure 2 is a sectional View taken along line 2 2 in Figure l. y

In a sense, the present invention contemplates the provision of an improved extrusion apparatus comprising an elongated hollow barrel formed of a magnetic material and having a feed end and a discharge end; means for conveying material from said feed end to said discharge end; means for delivering a thermoplastic material into said rbarrel at said feed end; an extrusion die located at the discharge end of said barrel; an electrical solenoid registering with said barrel and having an inner surface spaced between 1/s and 1A of an inch from the outer surface of said barrel to define a duct through which air may be circulated; and means connecting said solenoid to a source of alternating current at a frequency between l0 and 1000 cycles per second.

g Another feature of the present invention resides in the provision of an improved extrusion appm'atus compris* ing an elongated hollow ban-el formed of a magnetic material having a feed end and a discharge end; means for conveying material from said feed end to said discharge end; means for delivering a thermoplastic material into said barrel at said feed end; an extrusion die located at the discharge end of said barrel; an electrical solenoid registering with said barrel; said barrel being provided with a portion of enlarged cross section adjacent to its feed end thereof; said solenoid portion of enlarged cross section having a transverse aperture formed therein communicating with the interior of said barrel; and means connecting said solenoid to a source of alternating current at a frequency between l0 and 1000 cycles per second.

Referring now to the drawings, which illustrate a preferred embodiment of the present invention, the numeral 10 generally designates a longitudinally extending extruder barrel of hollow cylindrical contigui-ation which is formed of a magnetic material, such as, for example,

plastic material, generally in the form of granules or c ordinary carbon steel or a stainless steel of the magnetic type. The barrel 10 is provided with an inner smooth cylindrical surface 11 of circular transverse cross section and an outer cylindrical surface 12 likewise of circular cross section and is provided with a plurality of longitudinally extending wells `13 for housing 'tempera-i ture responsive elements Aas will be hereinafter set forth.

The-forward or discharge end of the barrel is of slightly reduced cross section and is threaded as at 14. An internally threaded annular iiange 16 engages the threaded portion and is provided with circumferentially spaced longitudinally extending tappedfopenings 17;V An extrusion die 18 of conventionalconstruction'isprovided with a longitudinally extending'extrusion opening 19 and a rearwardly directed conduit-20fprovidedfwitl1 a funnel shaped guide 21 directed to theextrusion opening 19. The conduit 2t) carries an outwardly directed flange 2.2 which is secured to the barrel ange 1"6vbyk means of screws passing'through openings formedin-the flange 221and engaged tapped openings-17 in the 'flange 16. Also carried by the conduit are the usuallbreakerplates 23 and screens 24. Y

The transverse cross sectionalarea of' theV barrel'y 10 is substantially increased' at the-feed endthereof bythe: provision of a metal sleeve 26 encircling the barrel 1,0"at

the feed endv thereof and in tight fitting abutment'with the outer surface 12 thereof. The cross-sectionalarea of the barrel at the feed end thereof,'includingthe barrel perl se 10 and the collar or sleeve-26,` should'preferablybe at least two to three times the cross-sectonalarealof'the barrel 10. A frustoconical opening 27 is formed in-the collar 26 and communicates by wayofanopening 28 formed in the barrel 10 with the interior of the barrel. A funnel shaped hopper 29 has its lower end` registering with and nesting in the frusto-conical opening-'27 and provides a means for feeding ythe plastic material, which-is usually in pellet or granular form, intothe interioroftheextruder barrel 10.

Disposed within the barrel 10 and extending coaxially therewith is a screw 30 of conventional construction including a leading torpedo-32 and'a'trailing'advancing and compression screw 33 of varying pitch which decreasesapproaching the leading end of the screw.Y Thus, gradually increased pressure is applied to the liquiedplastic as,

it advances along the length of the barrel 10. The screw 30, is provided with a shaft 34 projectingrearwardly through a suitable bushing and driven by theV conventional motivating means;

A plurality of solenoids 36 arranged in spacedend-tothe cooling of the barrel and whichtfurther heat-insulates the solenoids 36 from the barrel 10 to-keep the temperature of the solenoids relatively low. The end-to-'end spac-Y ingbetween the solenoids 36 likewise facilitates the circulation of air about the outerbarrel surface 12. The distance betweenthe confro-nting face of the solenoids 36 should be at least twice the'distance between-the solenoid and the barrel 10 and less than'thebarrel wall thickness. Each successive pair of solenoids 36 are connected in series in a similar sense; that is, the turnsof'the'solenoid extend in a common current carrying direction through the controlled winding 38 of a saturable1core'reactor39 to a source of alternating current 35 ata frequency between 10 and 1000 cycles per second. This-alternating current may be derived directly fromthe commercial power lines at 60 cycles per secondA or at the'prevailingV frequency provided that it falls within the above range. The saturable core reactor39 is provided with acontrol winding 40 connected ,to` the drectcurrent outputfof an amplier 41. y

One of the terminalsto'the input vof'th'e` amplifier41fis connected in series to the'resistonelementof apotentiometer 42' acrosswhich resistor'element is applied a suitable -D.C. standard "voltage: The Y othcrterminal of the 4 amplifier input 41 is connected by way of a thermocouple 43 disposed in a well 13 to the arm of the potentiometer 42. It should be noted that a thermostat or other temperature responsive elementk may be substituted for the thermocouple 43 and the circuit network correspondingly modified as is well known in the art.

The input and output of each amplifier 41 is so fixed that a rise-in the temperature of the corresponding temperature responsive ele1:nent43,-isA accompanied by a decrease in the current through the saturable core reactor controlV winding 40 to thereby increase the impedance of'the saturablecorereactorcontrolled winding 30 and decrease the current through thecorresponding solenoids 36.. Similarly, a dropinthe temperatureof-the element 43 is accompanied by an increase in the current through the control winding t40to decrease the impedance of controlled winding 39 and increase the current through the solenoids 36. By adjusting the potentiometer 42 or any equivalent circuit, the., temperature'of the corresponding zoneof the barrel ltlmay Abe adjusted.l It should be noted that other well known adjustable circuit networks mayv be employed to control the current through the winding 40 in proper response to the element 43.

The resonant frequency 'ofthe groups of solenoids 36 as connected is preferably higherthan the frequency of the applied current under'all operatingconditions. thermore, the controlled `winding ofthe saturable core reactors, when no signal is applieditol the controlledl winding thereof,A should preferably resonate at the frequency of the appliedl alternating current. The consequence of the above is a superiorcontrol ofthe heating ofthe barrel 10.

The Vthickness ofthe wallof barrel 10, for maximum efficiency, should beas vsmall asf'possiblecommensurate with the strength requirements of the barrel to withstand the pressure developedwithin the extruder and should prefcrablygnot'exceed approximately'40%v of the inner diameter of the barrel.' The advantages realizedby the present improved system for heating the extruder barrel are numerous and radical. The frequency of the applied alternating current and the thicknessof` the barrel 10 are such that intense heating 'is effected 'along the interior surface Vof the barrel 'and as a result the temperature gradient radially through the wall ofthe barrel 10Y is such that the heat ilow lis generally outwardly, the temperature within the barrelv 10 being normally higher than the temperature at theoutersurface of theV barrel. Thus, when the. frictional heat generated' by the screw 30 is greater than thatv requiredto maintain the desired-ternperatures of the extruded plastic, there is a rapid ilow of heat outwardly through the wall of the barrel `10 since the outer surface thereof is generallyat a lower-temperature than the inner surface thereof. and there isino-impeding heat insulating material.

Furthermore, when heat is required tobe delivered by the'heaters to the interior ofthe barrello, the heat `losses are likewise relatively low since the heat isV generated principally below the outer surface of the barrely10. This greatly contrasts with the conventional electrical heating wherein the outer surface is at maximum temperature during the heating cycle and heat lossesare much greater than with the present improved heating system. In addition, the timeY required to raise the temperature of the barrel 10. from room temperature toY normal operating temperature is muchless employing the present heating system thanv withthe conventional electric heating systems. The .powerrequirements for the regulated heating of an extruder barrel employing the present-improved system are-a smallfractionof thatrequired in conventional. extruder heating systems.

Thetrailing solenoid 36 iszjustforward oftheenlarged portion -of the barrel 10.de1"1ned vby thecol1ar.26, so that very little heat isinducedrinlthat'portion,of the barrel anterior to thetrailingsolenoid 36. Asa result, the only heat reaching the trailingend of the extruderis by way of conduction along the barrel 10. Inasmuch as the amount o'f heat contained in the barrel 10 is relatively low, the temperature of the collar 26 and the hopper 29 is insignificantly raised thereby permitting the free and unimpeded llow of the solid granular or pelletized material in the hopper 29.

In accordance with a speciiic example of the improved extruder, the barrel from the discharge end to the leading shoulder of the collar 26 is approximately 22 inches long and is formed of a steel tube having an outer diameter of 2% inches and an inner diameter of 11/2 inches. The outer diameter of the collar 26 is 51/2 inches and is approximately 51/2 inches long. There are provided eight solenoids 36, each solenoid having 400 turns, being approximately 21A inches long and 5/s inch thick, the inner surfaces thereof being spaced from the outer surfaces of the barrel 10 approximately 1/s inch and the confronting faces of the successive solenoids 36 being separated approximately inch. The leading and trailing seriesconnected pairs of solenoids are each provided with an individual control regulating circuit network as above described, whereas the intermediate two pairs of series-connected solenoids are connected in parallel and provided with a common current regulating network as above described.

In operating the above extruder in the extrusion of acrylic resins which requires a temperature of approximately 400 F., the three circuit networks were adjusted so as to provide the desired temperature distribution along the length of the barrel, the temperature of the barrel zone heated by the leading and trailing pairs of solenoids 36 being lower than that required by the intermediate zone of the extruder barrel. Under normal operating conditions, the voltage across the leading pair of solenoids is 150 volts and the current is normally 1% amperes. The voltage across the two intermediate pairs is 125 volts and the controlled current thereto 2 amperes, and the voltage across the trailing solenoids is 175 volts and 2 amperes. The total power consumption under normal operating conditions is under approximately 900 watts, a small fraction of that required with the conventionally heated extruders. The frequency employed was 60 cycles per second and the line voltage was 220 volts.

While there has been described and illustrated a preferred embodiment of the present invention, it is apparent that numerous alterations and omissions may be made without departing from the spirit thereof. For example, while the barrel 10 has been illustrated as formed of a single piece, it may include a liner in the usual manner. Furthermore, the arrangements of the coils may be varied in accordance with the requirements of the particular extruder and the temperature distribution adjusted to the optimum needs of the extruded plastic.

I claim:

1. An improved extrusion apparatus comprising an elongated hollow barrel formed of a magnetic material and having a feed and a discharge end, means 'for conveying material from said feed end to said discharge end, means for delivering a thermoplastic material into said barrel at said feed end, an extrusion die located at the discharge end of said barrel, a plurality of longitudinally spaced electrical solenoids registering with and magnetically coupled by said barrel and having inner surfaces spaced frorn the outer surface of the said barrel, the distance between the confronting faces of successive solenoids being less than the wall thickness of said barrel, and means connecting each of said solenoids to an independently continuously controlled source of alternating current at substantially the same phase and at a frequency between 10 and 1000 cycles per second and each said means including a saturable core reactor having a controlled winding connected in series with said respective solenoid and said source of alternating current and a control winding, and a temperature responsive element located on said barrel and means for applying to said control winding a direct current varying inversely as the temperature of said temperature responsive element, the` resonant frequency of said solenoids being higher than said alternating current frequency.

2. An improved extrusion apparatus in accordance with claim 1, wherein the distance between the outer face of said barrel and the confronting faces of said solenoids is at least l/s inch.

3. An improved extrusion apparatus in accordance with claim 1, wherein said barrel is provided with a portion of enlarged cross section at the feed end thereof and said solenoid extends to a point short of said portion of enlarged cross section whereby the temperature of said section is less than that of said barrel and including a. hopper communicating with said barrel at a point posterior to the leading end of said portion of enlarged cross section.

References Cited in the file of this patent UNITED STATES PATENTS 2,226,447 Smith et al. Dec. 24. 1940 2,309,496 Bird et al. Jan. 26, 1943 2,541,201 Buecken et al. Feb. 13, 1951 2,721,729 Van Riper Oct. 25, 1955 

